I have often thought that the mileage taken for the fuel gauge to noticeably drop below the 'Full mark', after filling the tank, can be quite variable - suggesting that the nozzles on the pumps are not set in a way that the nozzles click off at the same fuel level. If this is the case the method you suggest for estimating mpg is open to error. Can I suggest that people wishing to estimate mpg by filling the tank as high as possible, driving a suitable mileage and then taking the pump reading as indicative of the petrol consumption, should use the exact same pump at the beginning and end of the sampling period.